Light weight olefins have traditionally been produced through the process of steam or catalytic cracking. Because of the limited availability and high cost of petroleum sources, the cost of producing light weight olefins from such petroleum sources has been steadily increasing. Light weight olefins serve as feeds for the production of numerous chemicals. As the emerging economies of the Third World strain toward growth and expansion, the demand for light weight olefins will increase dramatically.
The search for alternative materials for light weight olefin production has led to the use of oxygenates such as alcohols and, more particularly, to the use of methanol, ethanol and higher alcohols or their derivatives. These alcohols may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids and from carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based routes for the production of olefins and other hydrocarbons. Methanol, in particular, is useful in this process which is referred to herein as methanol to olefins (MTO).
Molecular sieve catalysts such as the microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: U.S. Pat. Nos. 3,928,483; 4,025,575; 4,052,479; 4,496,786; 4,547,616; 4,677,243; 4,843,183; 4,499,314; 4,447,669; 5,095,163; 5,191,141; 5,126,308; 4,973,792; and 4,861,938.
The process may be generally conducted in the presence of one or more diluents which may be present in the oxygenate-containing feed in an amount between about 1 and about 99 mol-%, based on the total number of moles of all feed and diluent components fed to the reaction zone (or catalyst). Diluents include, but are not limited to, helium, argon, nitrogen, carbon monoxide, carbon dioxide, hydrogen, water, paraffins, hydrocarbons (such as methane and the like), aromatic compounds, or mixtures thereof. U.S. Pat. Nos. 4,861,938 and 4,677,242 particularly emphasize the use of a diluent combined with the feed to the reaction zone to maintain sufficient catalyst selectivity toward the production of light weight olefin products, particularly ethylene. The above U.S. patents are hereby incorporated by reference in their entirety.
U.S. Pat. No. 5,026,935 discloses a process for the preparation of ethylene from C4 or higher feed by the combination of cracking and metathesis to form ethylene and propylene and at least a portion of the propylene is metathesized to ethylene.
Generally, the ratio of ethylene/propylene on a carbon basis varies from about 0.1 to about 10 and, more typically, varies from about 0.8 to about 2.5. Ethylene and propylene are particularly desirable olefins but it has been found that their yields are reduced by the production of medium weight hydrocarbons such as C4, C5 and C6 olefins, as well as some heavier components. Methods are needed to alter the product distribution in the MTO process for making light weight olefins to provide processing flexibility and overcome the equilibrium limitations of alumino phosphate catalysts in the MTO process. Methods are sought to reduce the production of C4, C5 and higher olefins from the MTO process relative to the production of ethylene and propylene. Such yield improvements of ethylene and propylene significantly improve the economies of the methanol to olefins process. A new improved process for conversion of oxygenates to hydrocarbons is provided by the present invention in the enhancement of production of ethylene and propylene.